Forest management drives evolution of understorey herbs

Forest management has a strong impact on the forest structure and subsequently on the biotic and abiotic forest understorey environment. Forest understorey herbs can thus be expected to evolutionary respond to management-induced environmental variation (provided sufficient time for adaptation), but this has been little tested to date. Here we use a common garden, to test for genetically based variation in phenotypic traits in populations of forest herbs sampled along a forest management intensity gradient. Five different herbaceous species were sampled from 70-100 populations in three regions in Germany and were tested for genetically based variation in flowering start, proportion of flowering ramets, and plant height. Additionally, we investigated the effects of management-induced environmental variation and performed structural equation modelling to study how forest management drives trait differentiation via its effects on the microenvironment. We found that the studied forest understorey herbs varied genetically in the measured functional and phenological traits among the sampled populations. Forest management likely affected the traits in various directions and strengths depending on the species, either directly through variation in forest structural attributes or indirectly through changes in the microclimatic environment on the forest floor. We show that forest management can have evolutionary consequences for forest understorey plants. In an applied context, diverse forest management actions within landscapes thus creates heterogeneity that selects for different plant traits and thus helps conserving genetic diversity.